Slice the bread into 1" cubes, then spread 3/4's of the cubes evenly out in a 9x13" baking dish that's been sprayed with nonstick spray.
6 thick slices rye bread
Transfer the remaining bread cubes to the bowl of a food processor and pulse them until they're fine bread crumbs. Set aside.
Spread half the pastrami pieces evenly out over the bread cubes, pulling them apart as necessary.
1 lb thinly slice pastrami
Spread the sauerkraut evenly out over the pastrami, followed by the relish, then half the caraway seeds.
1 15 oz jar sauerkraut, 1 10 oz jar dill pickle relish, 2 tsp caraway seeds
Sprinkle half the cheese evenly out overtop of the casserole, followed by the rest of the meat. Using a spatula press the layers down a bit, only if needed.
4 cups shredded Swiss cheese
Sprinkle the remaining cheese and seeds evenly out overtop of the dish.
In a mixing bowl add the milk, eggs, dressing, and mustard and whisk together until evenly combined. Pour this mixture evenly out over the casserole, then use a spatula to spread it out as needed.
1 cup milk, 3 large eggs, 1/3 - 1/2 cup thousand island dressing, 1/4 cup yellow mustard
Sprinkle the rye bread crumbs evenly out overtop.
Cover the dish tightly with foil, then bake it at 350° for 45 minutes.
Remove the dish from the oven, uncover, and let it rest for 5-10 minutes.
Slice, serve, and enjoy warm!